1. Field of the Invention
The present invention relates to an acoustic noise absorbing device, and more particularly to a tunable resonant acoustic noise absorbing device.
2. Description of Related Art
Broadband acoustic liners are based on the mechanism of multiple cavity resonance. Bulk and cellular core structures are the most commonly used types.
The simplest acoustic liner is a single degree of freedom (SDOF) liner. The face sheet exposed to the sound is perforated and the bottom sheet is solid. SDOF designs are usually tuned to attenuate one octave and their tuning abilities are limited and fixed to one design point.
Multiple degree of freedom (MDOF) liners on the other hand are liners with multiple-layer sandwich construction. While multi-chamber liners are used to address broadband noise, SDOF are not broadband. The face sheet exposed to the sound is perforated and the bottom sheet is solid. Intermediate sheets or septum vary from perforates to screen-like material. MDOF designs are more complicated and are able to attenuate noise over a larger bandwidth. The current state-of-the-art in broadband acoustic liners is passive in implementation, fabricated with one attenuation design point.
One of the fundamental characteristics of each cavity or chamber within resonant liners is suppression of noise over a relatively narrow bandwidth. For SDOF liners, the useful bandwidth is approximately one octave. A MDOF liner has wider bandwidth than a SDOF liner. It is most effective when designed for two adjacent harmonics of blade passing frequency (BPF) or approximately two octaves. This is accomplished through selection of geometric properties for the chamber depth and face sheet and septum properties. The challenge to noise attenuation for example is suppressing noise generated by aircraft engines over several octaves.
There is a push in the industry to implement more dynamic technology in the liners making passive designs tunable. The Helmholtz Resonator concept is not new, and it has been documented extensively in the literature. However, the ability to accurately control the resonator dimensions, or volume throughout a variety of operating environments, is still in its infancy. One design of current Helmholtz resonators uses a compliant piezo-electric composite back plate. The piezo-electric material is placed in an RLC (resistive-inductive-capacitive) circuit. Compliance of the back plate is controlled through manipulation of this RLC circuit, allowing impedance of the electromechanical acoustic liner to be tuned in-situ. An interesting feature of this concept is that, although the Helmholtz resonator was constructed using a SDOF design, the compliant lower boundary had the effect of making the resonator behave like a MDOF design, broadening the effective frequency attenuation range. However, the two resonance frequencies produced by the piezo-electric back plate are not independently tunable.
Prior attempts at passive-adaptive noise control were made with a self-tuning Helmholtz resonator. This concept uses a resonator with a moving internal partition. The effect is to change the internal volume of the resonator. A tuning range of 100 Hz can be achieved with this design. Self-tuning can be enabled through a feedback loop microprocessor control system.
Not all adaptive designs feature physically moving or changing boundary conditions. Biasing air flow through the lining and changing the temperature of the air within the contained volume of the resonator has been used as methods to achieve active control. Biasing air flow seeks to change the impedance of the liner by affecting the impedance of the orifice. The effect on impedance of biasing airflow on the liner surface orifice is not as significant as it is for the septum orifice between two chambers as used in a MDOF design. Controlling the air flow through the septum showed the ability to tune the centering frequency of the resonator with desirable increases in impedance. Both positive and negative biasing airflows have been previously demonstrated. Sourcing of biasing airflow in the air intake of modern and future engine designs from bleed air will be seen as detrimental as it detracts from engine efficiency. The second option is to change the properties of the air inside the chamber by heating the air inside the camber. This adjusts the impedance through changing the normalization properties. Energy efficiency and small tuning range detract from the viability of this concept.
Passive liners are manufactured to a fixed design point, whereas active liners can change their design point post manufacture to suit multiple operating conditions. All acoustic liners currently in use in jet engine liners were found to be passive. The ability to control the acoustic impedance in-situ of a liner would provide a valuable tool to improve the performance of liners.
The ability to implement adaptive acoustic engine liners would be a benefit as the operating conditions of aircraft engines are variable, based on flight conditions and aircraft trim. Compromises are usually made in the design of passive liners in order to reach a design that is a best fit for multiple operating conditions. Adaptability of liner design in-situ will allow for a liner that is more capable of meeting Federal Aviation Association (FAA) mandated noise standards, for conditions such as approach, cutback and sideline.